To understand the biophysical basis of the diffusion MR signal, Uri Nevo has successfully constructed and tested an experimental system for interrogating organotypic cultured brain slices using diffusion MRI methods. This work has already shown promising results relating changes in the measured diffusion coefficient map to changes in environmental conditions to which the cultured tissue is subjected. A theoretical aspect of this work is the development of model systems in which we can demonstrate how microscopic flows manifest themselves as "pseudo-diffusion", resulting in additional signal loss in diffusion weighted MRI experiments. In the area of Transcranial Magnetic Stimulation (TMS), Pedro Miranda and his group in Lisbon, in association with STBB, has performed detailed calculations using finite element methods (FEM), to predict the electric field and current density distributions induced in the brain during TMS. Previously, we found that both tissue heterogeneity and anisotropy of the electrical conductivity (i.e., the conductivity tensor field) contribute significantly to distort the induced fields, and even to create excitatory or inhibitory hot spots in some regions. These phenomena could have significant clinical consequences both in interpreting or inferring the region or locus of excitation and in determining the source of nerve excitation. More recently, we have focussed on determining the dominant physical mechanisms responsible for cortical excitation. A longer term goal is to marry our macroscopic models of TMS with microscopic models of nerve excitability in the CNS. More detailed FEM models of TMS in the cortex are under development.